David Cameron is to meet the president of Iran to urge him to join an international effort to tackle the Islamic State (IS) terror group and drop his country's support for the Assad regime in Syria.

The meeting with Hassan Rouhani, at a crucial United Nations summit in New York, will be the first time a British prime minister has held face-to-face bilateral talks with an Iranian president since the country's Islamic revolution in 1979.

It marks Mr Cameron's determination to enlist the active support of regional powers in the Middle East in taking on IS, which has seized control of swathes of territory in Iraq and Syria and carried out a string of atrocities against local people and foreigners as it seeks to impose its own brand of extremist Sunni Islam.

The Prime Minister hopes to secure UN approval for a comprehensive strategy to deal with the brutal militant group, and will join US president Barack Obama and other members of the UN Security Council to discuss the threat posed by foreign fighters - thought to include hundreds of Britons - engaged in the conflict.

He will use an address to the UN General Assembly to call on the whole world to come together to offer broad-based support for the new inclusive Iraqi government led by Haider Abadi in Baghdad and to defeat the threat from extreme Islamist groups such as IS, also known as Isis or Isil.

Mr Cameron's visit to New York - which will also see him attend UN secretary general Ban Ki-moon's Climate Summit and hold discussions on US-EU trade - comes amid continued concern for the safety of British hostages Alan Henning and John Cantlie, who have featured in recent propaganda videos released by IS. The group has already beheaded British aid worker David Haines as well as US journalists James Foley and Steven Sotloff.

Britain has been helping supply arms to Kurdish peshmerga fighters taking part in the struggle with IS, as well as offering support to Mr Abadi's government, and it was a driving force behind a UN Security Council presidential statement on Friday calling on the international community to help Iraq defeat the extremists. But Mr Cameron has ruled out putting British soldiers' "boots on the ground" and it is unclear whether the UK will join the US and France in air strikes against Islamic State forces.

The PM has said he would want to consult the House of Commons, if possible, before engaging British forces, raising the prospect of a possible recall of Parliament after his return from the US on Thursday.

President Obama earlier this month announced a broad battle plan to "degrade and destroy" IS, including the expansion of air strikes into Syrian territory, 1,600 US military advisers on the ground in Iraq and increased support for moderate Syrian groups to fight the militants. He is hoping to secure support from a wide range of countries, including key powers in the Arab world, at this week's UN gathering.

Despite an earlier indication from Supreme Leader Ayatollah Ali Khamanei that he had rebuffed overtures to work with the US and its allies, reports this weekend suggested that Tehran was sending signals of its readiness to play a part in taking on IS.

Tehran is also calling for flexibility on its uranium enrichment programme, which has sparked sanctions because of Western concerns that any nuclear capability may be used for military purposes. Following talks with his Iranian counterpart on Sunday, US secretary of state John Kerry held out the prospect of progress in long-stalled negotiations.

Downing Street said that Mr Cameron had "no illusion" about the threat posed by Iran's nuclear operations, but was ready to work with Tehran if it was willing to join the effort to defeat IS.

The Prime Minister and Mr Rouhani will discuss the role Iran can play in helping to tackle Islamic State and to support the new Iraqi government. But Mr Cameron will stress that IS cannot be tackled in Iraq alone, and will call on the Iranian leader to drop support for president Bashar Assad, who he believes has created the conditions which have allowed terrorism to flourish in Syria.

In a message on behalf of the E3+3 group - the UK, US, France, Germany, Russia and China - he will tell Mr Rouhani that Iran must " show flexibility and be realistic about the future scope of its nuclear programme, in particular on the issue of enrichment".

A Downing Street source said: "We are under no illusion about the dangers of Iran's nuclear programme and our approach on that is not changing.

"However, if Iran is willing to join the international community to defeat Isil, then we will work with them on that but will be clear that you cannot take one approach in Baghdad and another in Damascus. You need a political solution in both if you are serious about defeating Isil."

In talks with Egyptian president Abdel Fattah el-Sisi, Mr Cameron is expected to say that the UK wants to work with Cairo on tackling the extremist threat in the region, but caution that this can only happen in the context of inclusive governments, strong institutions, safeguards for minorities and respect for human rights. The leaders are also expected to discuss Libya, where the PM will call for regional support behind international efforts to secure a political settlement to ongoing instability.

The Prime Minister will kick off his visit to New York by joining US business leaders to launch a campaign for the conclusion in 2015 of a comprehensive EU-US trade deal, which he believes could be worth £10 billion to the UK economy.

Mr Cameron will offer to convene a meeting of EU and US chief executives, alongside key European political counterparts, in Brussels to maintain momentum behind the deal.